1. Field of the Invention
The present invention provides a process for the preparation of polyether polyols with equivalent molecular weights of from 8,000 to 20,000 g/mol, preferably from 9,000 to 20,000 g/mol, particularly preferably from 10,000 to 16,000 g/mol.
2. Description of Related Art
Polyether polyols which are suitable for the preparation of polyurethanes can be obtained via various preparation processes. The base-catalysed addition of alkylene oxides on to H-functional starter compounds on the one hand and the use of double metal cyanide compounds as catalysts (“DMC catalysts”) for the addition of alkylene oxides on to H-functional starter compounds on the other hand are of large-scale industrial importance. The addition of alkylene oxides on to suitable starter compounds catalysed by (Lewis) acids is of minor importance.
Under alkali metal hydroxide catalysis, undesirable side reactions increase significantly with increasing molecular weight of the polymer. There is to be mentioned here in particular the isomerization of propylene oxide to allyl alcohol, which at high equivalent molecular weights (or low OH numbers) leads to a high content of monofunctional polyether species in the reaction mixture and therefore to a significant lowering of the functionality. The monofunctional polyether molecules therefore have an adverse effect on the full curing properties and the profile of physical properties of polyurethane systems and other materials which can be prepared from these polyethers, such as, for example, silane-based sealants.
By the use of DMC catalysts it has become possible to speed up the addition of alkylene oxides, in particular propylene oxide, on to H-functional starter compounds down to very low OH numbers, i.e. high equivalent molecular weights, without the abovementioned isomerization of propylene oxide to allyl alcohol occurring to a noticeable extent. Highly active DMC catalysts, which are described, e.g. in U.S. Pat. No. 5,470,813, EP-A 700949, EP-A 743093, EP-A 761708, WO-A 97/40086, WO-A 98/16310 and WO-A 00/47649, furthermore have an exceptionally high activity and render possible polyether polyol preparation at very low catalyst concentrations (100 ppm or less), so that it is no longer necessary to separate off the catalyst from the finished product. The highly active DMC catalysts described in EP-A 700949, which, in addition to a double metal cyanide compound (e.g. zinc hexacyanocobaltate(III)) and an organic complexing ligand (e.g. tert-butanol), also contain a polyether with a number-average molecular weight of greater than 500 g/mol, are a typical example.
The equivalent molecular weight of materials containing active hydrogen atoms is to be understood as meaning the total weight of the material containing active hydrogen atoms divided by the number of active hydrogen atoms. In the case of materials containing hydroxyl groups (such as, for example, polyether polyols), it is related to the OH number (hydroxyl number) as follows:Equivalent molecular weight=56,100/(OH number [mg of KOH/])  (I)
The equivalent molecular weight of the polyether polyol is thus determined according to formula (I), the hydroxyl number of the polyether polyol being determined in accordance with DIN 53240.
The DMC-catalysed preparation of alkylene oxide addition products with high equivalent molecular weights and the use thereof for the preparation of polyurethane- or polyurea-based materials are known to the person skilled in the art. For example, DE-A 4117679 and U.S. Pat. No. 5,096,993 disclose the use of polyhydroxy and polyamine compounds with molecular weights of up to 30,000 Da for the preparation of flexible polyurethane or polyurea elastomers by the reaction injection moulding process (“RIM” technology). WO-A 9104997 discloses polyether triols with molecular weights of up to 30,000 Da as the polyether component of isocyanate-terminated prepolymers which are employed in high performance polyurethane sealant systems. EP-A 1316573 discloses a process for obtaining flexible foam bodies with good long-term use properties which can be rapidly removed from the mould, for the preparation of which polyether polyols prepared via DMC catalysis with equivalent molecular weights of from preferably 5,000 to 11,000 Da are employed as the polyol component. EP-A 0425694 discloses isocyanate-terminated polyether prepolymers, the polyether component of which has equivalent molecular weights of up to 15,000 Da. Such polyether polyols are obtained via DMC catalysis. Polyethers prepared via DMC catalysis with equivalent molecular weights of up to 15,000 Da are used in EP-A 0732561 as starting compounds for the preparation of moisture-curing sealant systems based on polymers containing silane groups.
In the DMC-catalysed preparation of polyether polyols with very high equivalent molecular weights (8,000 Da or higher), achieving narrow molecular weight distributions and, associated with this, manageable viscosities presents increasing problems as the equivalent weight increases. The processes of the prior art offer no solutions to this problem.